Aggregation of activated platelets on ruptured or eroded atherosclerotic plaques initiates thromboses of the arterial system, resulting in ischemic syndromes. The propensity of platelets to aggregate in vivo can be characterized by in vitro assays and used to identify individuals with hyper-aggregable platelets at risk then for myocardial infarction, stroke, and peripheral arterial occlusions. These platelet function assays are moderately to highly heritable supporting the hypothesis that genetic variations underlie individual variability in the tendency for arterial thrombosis. During the past seven years, in an ongoing NHLBI-funded study called GeneSTAR (Genetic Study of Aspirin Responsiveness), a genome-wide association study (GWAS) revealed multiple common genetic loci that pass stringent GWAS thresholds in African American and European Americans families at high risk for CHD. Common variants were found to determine variability in native platelet aggregation as well as residual platelet aggregation after low dose aspirin (ASA) intervention. However, collectively the loci identified through this common variant approach account for less than 35% the total heritability of these phenotypes in the GeneSTAR families. In this study we aim to extend our family-based GWAS design in an integrative approach to: 1) identify rare variants in genes that are associated with native and residual post- ASA platelet aggregation, testing the hypothesis that a significant fraction of the 'missing heritability' in platelet aggregation phenotypes (i.e. that not explained by the common GWAS signal) is due to these rare variants; and 2) follow up on the GWAS-identified loci to determine the underlying 'causal' variants tagged by the GWAS association signal. In a family-based exome sequencing approach we will sequence 200 hyper-aggregable individuals selected from African American and European American GeneSTAR families with clustering of platelet aggregation. This data will be leveraged against a public catalog of exome variation in the NHLBI-funded Exome Sequencing Project to identify genes enriched for rare variants associated with platelet hyper aggregation. Validated exome sequencing-identified genes along with the GWAS-identified loci will be followed up relying on a targeted deep resequencing approach of 1,300 African American and European American subjects from additional GeneSTAR families. The results from this integrative GWAS and exome approach will lead to a better understanding of the role of genetic variants (common and rare) in the determination of platelet aggregation native and residual post-ASA, including possible racial differences, and should enable genotypic tailoring of preventive therapy for CHD in high-risk individuals. PUBLIC HEALTH RELEVANCE: Native and residual post aspirin platelet hyper aggregation, a strong risk factor for ischemic syndromes, is moderately to highly heritable. Our preliminary data using a genomewide association study (GWAS) in African American and European American families suggest high 'missing heritability' (i.e. that not explained by the common GWAS signal detected). The primary hypothesis is that genes harboring rare genetic variants determining platelet aggregation account for a substantial fraction of missing trait heritability, and an integrative family-based approach of GWAS and exome-sequencing will be applied to test this hypothesis.